Hepatitis C virus (HCV) infection is often associated with chronic liver disease, sometimes resulting in cirrhosis and hepatocellular carcinoma. The lack of in vitro systems for HCV propagation has hampered biological studies on the virion and its mechanism(s) of cell entry, and the cellular receptors remain unknown. The selective association of a virus with a target cell is usually determined by an interaction between the viral glycoproteins (gps) and specific cell-surface receptor(s) and is an essential step in the initiation of infection. Such interaction(s) often define the host range and cellular or tissue tropism of a virus and have a role in determining virus pathogenicity. HCV encodes two envelope gps E1 and E2, which accumulate in the endoplasmic reticulum, the proposed site for HCV assembly and budding. In the absence of native HCV particles, truncated version(s) of E2 and virus-like particles expressed in insect cell systems have been used as mimics to study virus-cell interactions. Soluble versions of E2 have identified interactions with CD81, scavenger receptor class B type 1 and DC-SIGN. However, these studies only measure HCV gp-cell attachment and not virus mediated cell fusion. To overcome the lack of a conventional cell culture system for the propagation of infectious HCV particles, we have developed retroviral pseudotypes which incorporate native HCV gps. These pseudotypes are infectious for human liver-derived cell lines and their infectivity is pH-dependent and neutralized by monoclonal antibodies specific for E2 and CD81. HepG2 cells can be rendered permissive for HCV pseudotype infection when engineered to express CD81, demonstrating that CD81 is a component of the receptor complex. This system will allow us to study HCV gp interaction(s) with target cells and to identify the cellular molecules involved. In addition, we will address whether neutralizing antibodies are elicited during HCV infection and whether they correlate with resolution or control of disease. [unreadable] [unreadable]